This invention relates to improved apparatus for perforating subsurface earth formations, and more particularly to a high density shaped charge perforating apparatus having a relatively small diameter.
It has become common practice in the completion of oil and gas wells to perforate the well casing and the surrounding formations to bring a well into production. One typical perforating device comprises detonating explosives of high energy and of the general character and form known as "shaped charges." In a typical embodment, a plurality of shaped charges are mounted in a fluid-tight, cylindrical housing or on an elongated bar member which is adapted to traverse a well to be perforated. The shaped charges are mounted in a variety of patterns along the length of the carrier member, with the axis of perforation directed generally laterally therefrom. A plurality of carrier members may be mechanically serially linked together to provide for perforating the casing and surrounding formations over a desired vertical interval.
In perforating subsurface formations it is often desirable to maximize the number of perforations in a given vertical interval of a well. Maximization of the number of perforations requires a high shot density perforating gun. High shot density perforating has significant operating time reduction over conventional perforating operations, thus reducing costs associated with the operation.
Typical high shot density perforating guns employ an array of shaped charges at a plurality of spaced intervals along the longitudinal length of the perforating gun. Each array typically utilizes three or four shaped charges with each array spaced three to four inches apart. While high shot density perforating guns of these designs have proven successful in larger diameter guns they are unsuited for smaller diameter guns.
Small diameter, four inches or less outer diameter, perforating guns are not suited for using an array of shaped charges. To employ such an array requires a significant reduction in the size of the shaped charges, thereby significantly reducing the amount of explosives, resulting in a reduction in the length of the perforation. An additional difficulty in high shot density, small diameter perforating gun is that of charge interference. Charge interference is the disturbance of the order of the undetonated charges by the explosion of a detonated charge. To avoid charge interference the detonator cord must set off a charge before the explosion of a previous charge interferes with the subsequent charge.
These and other disadvantages are overcome with the present invention by providing an apparatus for perforating well casings and the surrounding formations using a relatively small diameter perforating gun having a high shot density.